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18
Introduction
Table3.Actualvaluesofcoefficientofheatrecuperationεcalculatedfortypicaltemperaturesof
heatedairTa;ofexhaustgasesflowingfromcombustionchamberTfj:1300
oCandoftheair
suckedfromtheenvironmentTali:15
oC
Ta,i
Tfj
Ta
ε
1040
1300
15
0.8
oC
oC
oC
1300
450
15
0.33
oC
oC
oC
Theabovedatashowthatfortheefficiencyofenergyrecuperationofex-
haustgases
ε
toexceed80%,theairfedintotheburnershouldbeheatedtoat
least1040
oC.Heatingittoonly450oCassuresefficiencyofnomorethan33%.
InthefurnaceshowninFig.2,theheatingoftheairtohightemperaturesre-
sultedintheloweringofheatconsumptionforonetonofsteelfrom55Nm
3of
gasto35Nm
3.SinceinEuropealoneover100milliontonsofsteelarepro-
ducedannuallywithsteelheatedseveraltimesintheprocess,thepotential
savinginnaturalgasconsumptionmayexceedevenonebillionNm
3peryear.
Fortypicalgases,suchasnaturalgaseswithvaryingcontentofinertgas,
e.g.nitrogen,thecalculatedvalues
ε
areshowninFig.9.
Thestreamofheatexchangedbetweenexhaustgasesandtheenviron-
ment,forinstancethewallsofcombustionchamber,canbedescribedwith
Newton'sformula
Qk=Aα(Texh-Tw)
whereAistheaverageareaofheatexchange,αheattransfercoefficient,Texh-
thetemperatureofexhaustgasesandTwthetemperatureofthewallsofcom-
bustionchambers.
Coefficient
α
canbedeterminedbyclassicalformulaedescribingNusselt's
number
Nu=αD/λ
whereλstandsforcoefficientofheatconductivityofpostflamegasescalcu-
latedfortheaveragetemperatureofthegasesandwalls.Drepresentsaspe-
cificdimension,e.g.thediameterofthecombustionchamber.Nusselt'snum-
bercanbecalculatedfromformulaeobtainedfromthetheoryofsimilarityand
